1. Field of the Invention
The present invention relates to an electronic ballast circuit and, more specifically, to a circuit for detecting an end-of-life or false lamp condition for a fluorescent lamp driven by electronic ballast.
2. Description of the Related Art
When a fluorescent lamp is driven at high-frequency with an electric ballast, it is desirable to detect an end-of-life (EOL) lamp fault condition, or the operation of a lamp which is different from that which the ballast is designed for (i.e., a "false lamp"), and to shut the ballast off upon the occurrence of either event.
As a lamp approaches end-of-life, the voltage drop over one or both lamp filaments gradually increases, causing the total lamp voltage to increase symmetrically or asymmetrically. Similarly, if a false lamp is driven, the lamp voltage can exceed that which the ballast output stage is designed for. In either case, the increase in lamp voltage can cause the power being drawn by the lamp filaments to increase, depending on the type of output stage configuration connected to the lamp. If the filament power exceeds the maximum power for which the lamp is designed (i.e., the maximum power in the manufacturer's specifications), the heat being dissipated by the filament(s) can melt the tube glass, resulting in the fluorescent lamp falling out of the fixture and causing an injury.
FIG. 1 shows a typical ballast output stage that consists of a half-bridge driver circuit driving a totem-pole MOSFET or IGBT configuration at a given frequency. The square-wave voltage produced by the half-bridge switches drives a series-parallel lamp resonant circuit and therefore establishes the operating point for the lamp. The square wave voltage can be adjusted by changing the operating frequency and/or the DC bus voltage. Should the lamp voltage increase due to an end-of-life condition, the filament current, or capacitor C.sub.R current, also increases and is given by: EQU I.sub.Filament =C.sub.R .multidot.V.sub.LAMP .multidot.f.sub.run[ 1]
where:
C.sub.R =the capacitance of the resonant capacitor [in Farads]; PA1 V.sub.LAMP =the running lamp voltage amplitude [in Volts]; and PA1 f.sub.run =the running frequency [in Hertz].
Equation [1] shows that, for an increase in lamp voltage, there is a corresponding increase in the filament current. The filament power is then given by: EQU P.sub.Filament =(I.sub.Filament).sup.2 (R.sub.Filament) [2]
Equation [2] shows the power in the lamp filaments increasing quadratically with filament current. Equation [2] can also be written as: ##EQU1## which shows the filament power increasing quadratically with lamp voltage.
FIG. 2 shows a timing diagram for typical running voltages and currents corresponding to the ballast output stage for both normal and end-of-life (symmetrical and asymmetrical) operating conditions. The timing diagram of FIG. 2a shows an increase in filament current (I.sub.CR) during a symmetrical or asymmetrical increase in lamp voltage (V.sub.LAMP) during end-of-life. It is this increase in lamp voltage as the lamp ages which causes excessive power to be dissipated in the filaments (equation [3]).
FIG. 3 shows a typical prior art circuit for detecting both symmetrical and asymmetrical peak lamp voltage. If the lamp voltage increases to between approximately 30 and 50 volts above the nominal, the resulting signal V.sub.EOL can be compared against a threshold, for example, and the ballast shut down when the threshold is exceeded. However, it is difficult and expensive to monitor the voltage out at the lamp and then regulate the operation of the electronic ballast driver circuit based on that voltage.
Furthermore, because of possible asymmetry of the lamp voltage, present circuit solutions may include one or more capacitors to block any dc offset, rectifiers and filters for establishing a low-voltage signal representative of the lamp voltage, and high-voltage resistive dividers for sensing. Care must also be taken to ensure that other operating points of the ballast, such as start-up, pre-heat and ignition, do not conflict with the end-of-life circuit, therefore requiring additional circuitry.